1. Field of the Disclosure
The present disclosure relates generally to wireless networks, and more particularly, to a method of improving the availability of circuit switched services to a user equipment (UE) during abnormal scenarios.
2. Description of the Related Art
The third generation partnership project (3GPP) has developed universal mobile telecommunications system (UMTS) to meet the growing demands of mobile and Internet applications. As shown in FIG. 1, a UMTS network architecture 100 includes a UE 102, and a public land mobile network (PLMN), which is divided into a core network (CN) 106 and a radio access network (RAN) 104. The CN 106 is subdivided into a circuit switched (CS) service domain and a packet switched (PS) service domain. The UE 102 is configured to provide both CS and PS services. The CS service domain supports circuit switched services, such as, for example, voice, text etc. The PS service domain supports packet or Internet-based services, such as, for example, streaming movies, voice over Internet protocol (VOIP) calls, video calls, and the like, in the form of data packets. The RAN 104 may be embodied as a UMTS terrestrial RAN (UTRAN). The UE 102 communicates with the UTRAN 104 network using a radio resource connection (RRC). Only one RRC connection can be established between the UE 102 and the core network (CN) 106 at a time. In UMTS, the CS service domain includes a mobile switching center (MSC), for managing voice-based services, and a visited location register (VLR) database, for storing a location of the UE 102 within a serving network. The PS service domain includes a serving general packet radio service (GPRS) support node (SGSN) for handling several mobility and routing dependent functions that are required for PS-based services. The CN 106 also includes a home location register (HLR) where all PS location information, CS location information, and subscriber-related information are stored. The UE 102 and the UTRAN 104 communicate via a UU interface, whereas the UTRAN 104 and CN 106 communicate via a IU interface.
The UE performs a location area update (LAU), which is a regular procedure for informing the core network of the current location of the UE. The UE also performs an LAU in response to any of the conditions specified in technical specification (TS) 24.008, section 4.4.1. However, the LAU procedure may fail due to one or more conditions, such as, for example, location update reject with cause #17 network failure for maximum attempts, poor signaling connection with the network, random access failure, radio resource (RR) connection failure, T3210 timeout etc., as described in TS 24.008 section 4.4.4.9. One such scenario in which the LAU procedure fails is illustrated in FIG. 2.
Referring to FIG. 2, it is assumed that radio access bearers (RABs) are established between a UE 202, a UTRAN 204, and a MSC 206 of a CS domain and an SGSN 208 of PS domain of the core network, and a radio access bearer is established between the UE 202 and the UTRAN 204 at step 212. The UE 202 establishes a PS signaling connection 210 with the network (PMM_connected) at step 214. As shown in FIG. 2, the UE 202 encounters a trigger due to TS 24.008 section 4.4.1 at step 216, and sends a CS registration request using a location area update request (LAU_REQ) to the MSC 206 of the core network at step 218A. However, the UE 202 cannot avail itself of the services from the CN and the LAU has failed. The UE 202 waits for a time interval, which is specified by a timer T3211, to resend the LAU request to the MSC 206. However, the UE 202 is only permitted to resend the LAU request a maximum of ‘n’ times. The value ‘n’ depends on the latest versions of the 3GPP 24.008 specifications. (steps 218A-218N and steps 220A-220N) Upon the nth attempt to resend the LAU request, the RR connection or CS signaling connection between the UE 202 and the MSC 206 of the CN is released at step 222. When the CS signaling connection is released, the UE 202 fails to establish a voice call. Under this scenario, the UE 202 is permitted to perform PLMN selection to avail itself of the CS services from the selected PLMN network. However, it is not possible for the UE 202 to perform PLMN selection since only the CS connection was released and the PS signaling connection remains ongoing with the SGSN 208 of the CN. Therefore, the UE 202 cannot avail itself of CS services as long as the PS signaling connection is maintained by the PS core network.